PRINCIPLES OF TOXICOLOGY

The structure of the eye is shown in Figure 8.2. The cornea and conjunctiva are the first line of
defense against chemicals that contact the eye. Acids and alkalis are the more commonly encountered
agents that cause eye damage. Acids cause protein damage, which leads to eye injury that can range
in severity from burns that heal completely to those that perforate the globe. Alkalis such as ammonia
can also cause serious eye burns. Alkali burns differ from acid burns in that they may lead to additional
damage as time elapses, even if the burn was relatively mild at the time of injury. The best treatment
for both types of substances is irrigation with large volumes of water, which reduces the acid or alkali
concentration. Some compounds such as unslaked lime, which is found in many commercial wall
plasters, may stick to the eye and form clumps that are not readily diluted or washed away with water
irrigation. In these cases, irrigation followed by debridement of any remaining particles is required to
remove as much contamination as possible.
Two other agents that are frequent causes of eye damage are organic solvents and detergents.
Organic solvents cause damage by dissolving fats in the eye. The damage is seldom extensive or
long-lasting; however, if the solvent is hot, thermal burn may complicate the picture. Detergents act
by disrupting proteins in the eye and lowering the surface tension of aqueous solutions. Detergents
contain a nonpolar section and a polar section on the same molecule, allowing them to emulsify
compounds with widely different hydrophobicities. They are commonly found in wetting agents,
antifoaming agents, emulsifying agents, and solubilizers.
Other parts of the eye can be affected by chemicals, either directly or as a result of the ensuing
immune response that follows chemical burns. Many corrosive chemicals can cause lid damage and
scarring of the puncta or canaliculi. Normal tear flow enters the lacrimal canaliculi in the lid margin
via the puncta. The tears flow through the common canaliculus, lacrimal sac, and nasolacrimal duct
into the nasopharynx. Damage of the puncta or canaliculi obstructs tear flow and can cause the tears
to run down the cheeks. If a corrosive chemical penetrates the cornea and reaches the anterior chamber,
it may cause damage to the iris. Damage to the iris increases vascular permeability with ensuing
liberation of protein into the normally low-protein aqueous humor. These proteins can clog the outflow
of fluid from the interior of the eye and lead to pressure buildup and glaucoma. Leukocytes may also
infiltrate the aqueous humor from the inflamed iris vasculature and contribute to the blockage of the
outflow system.
Methanol is a unique eye toxicant since it affects the nerves of the eye and retinal and photoreceptor
cells. Ingested methanol is metabolized to formaldehyde, then formate, then CO 2 and water, with
formate considered the toxic metabolite. Methanol intoxication can lead to appreciable, and sometimes
permanent, loss of vision. Since methanol is first metabolized by alcohol dehydrogenase, ethanol can
be used to prevent the formation of formate. The ethanol successfully competes for the alcohol
dehydrogenase enzyme preventing the metabolism of methanol. Ethanol must be administered for a
sufficient length of time so that all the methanol is eliminated from the body.
8.4 SUMMARY
8.4 SUMMARY 167
Toxicity of the skin and eye can occur after exposure to many different substances that cause injury
through a variety of mechanisms. This chapter covered the major problems caused by chemicals
encountered at home and at work, but a variety of other skin and eye diseases, including the ones
mentioned in this chapter, can occur in reaction to systemically administered drugs. Whether a chemical
can produce an effect after it comes into contact with the skin or eye depends on many factors, including
genetic makeup, status of health, and efficiency of the skin’s barrier function. The following are some
important points about eye and skin toxicity.
• The most common skin disease is irritant and allergic contact dermatitis, with allergic contact
dermatitis usually being more severe.